1. Field of the Invention
The present invention relates to a document reading device for reading an image of a document while transporting the document.
1. Description of Related Art
In recent years, a document reading device, which automatically transports a sheet document inside the device and reads an image of the document at a predetermined position, is typically mounted to an image forming device and other devices. With such a document reading device, the image forming device becomes capable of performing image reading and image forming operations in efficient manner. Further, with advanced digital technology, such processes as reading document, converting an image data into electronic data, forming an image from the electronic data, etc., can be performed at higher speed, and the document reading device becomes capable of processing a greater number of sheets of document at once, so as to perform reading processing of more sheets of document at a high speed. In addition, enhanced reading performance and improved document transport technology allow the document reading devices to read various kinds of documents.
As one example of the above document reading device, Japanese Unexamined Patent Publication No. 2000-261625 (Tokukai 2000-261625; published on Sep. 22, 2000) discloses a document reading device for reading an image of a document while transporting the document, wherein the position of an exposure member for use in reading a document image by optical reading means is adjusted according to a thickness of a sheet document, so as to read the document image at the position where the document contacts a document platen.
Incidentally, in the case of reading an image of a document while transporting the document as described above, the document reading device reads an image of a document in the state where the exposure member stops at a determined position, i.e., with the optical reading means at a stop. Therefore, the document reading device generally has transparent support means (known as platen glass, contact glass, or the like), provided between the document and the exposure member, which serves as a reference in keeping a distance between the document and the exposure member constant.
With a greater variation in kinds of documents that can be processed by the document reading device, and improved reading performances, it becomes general to process documents written in a ballpoint pen, a pencil, or documents corrected in white-out by the document reading device. Therefore, the document reading device for reading an image of a document while transporting the document often faces the problem that the support means is stained with rubbed ink, powders of a pencil lead, white-out not completely hardened, and others, when the document is being transported over the support means, which adversely affects the reading operation by the optical reading means, resulting in lower quality of the read image. Furthermore, since the optical reading means and the support means do not move, if such stains appear in a read area of the support means where reading is carried out by the optical reading means, the problem of missing some part of read data and black lines would occur for each document, resulting in a significant deterioration in quality of the read image.
Further, with a certain distance above the support means provided is a transport path forming member which forms a document transport path with the support means in the vicinity of a reading position. This transport path forming member guides a document passing over the reading position to be transported along a flat-plate face of the support means, to regulate movement of the document in the direction orthogonal to the flat-plate face of the support means. The transport path forming member also serves as a background member for returning the reflected light to the optical reading means in an area where the document is not present. In response to the above problem of stains appeared on the support means, it may be arranged so as to increase the distance between the support means and the transport path forming member to 1.0 mm to 1.5 mm to resist rubbing of a document against the support means. However, even although the optical reading means, which is a reduced optical system, have a great depth of focus, the document reading device in which the distance between the support means and the transport path forming member is increased to 1.0 mm to 1.5 mm tends to cause more deterioration in read image quality due to insufficient area of the focus position, compared with the document reading device in which the above distance is set with a range of 0.5 mm and 1.0 mm.